(1) Field of the Invention
The present invention relates to a circuit for detecting humidity, and particularly to an improved circuit for humidity detection for detecting finishing condition of a foodstuff to be dielectrically heated in and by a heat-cooking apparatus such as, for example, an electronic range.
(2) Description of the Related Art
In prior art heat-cookers such as electronic ranges, finishing condition of a foodstuff cooked is controlled by a known means in which for example an absolute humidity detector which is used to detect vapor generated from the foodstuff with heat. The detection is placed in the vicinity of an exhaust port of a cooking chamber.
FIG. 1 shows a conventional circuit for detecting humidity, wherein elements, for example, platinum resistances, having positive temperature coefficients are used as temperature sensing elements for the detector mentioned above. In the configurations of a humidity detector for detecting an absolute humidity, as shown in FIG. 1, the circuit includes two temperature sensing elements Rs and Rr. The first temperature sensing element Rs is exposed in the atmosphere, while the second temperature sensing element Rr is hermetically sealed. Both the first and second temperature sensing elements Rs and Rr are heated by themselves at approximately equal temperatures, and the first temperature sensing element Rs detects a change of heat transfer coefficient due to a temperature variation of the atmosphere by detecting a change in its own resistance value, while the second temperature sensing element Rr compensates for the temperature variation of the atmosphere. In addition, the circuit is supplied with a power supply of a source voltage Eo for .activating the humidity detector, and includes two resistances, namely first and second resistances R1 and R2 which, together with the temperature sensing elements form a bridge circuit for detecting a ratio between the temperature sensing elements Rs and Rr, and a voltage converting circuit (which will be described hereinafter) for converting the ratio between the temperature sensing elements Rs and Rr into a voltage. Specifically, the aforementioned two temperature sensing elements Rs and Rr are arranged in series, one terminal of which is connected to the power supply of source voltage Eo, while other terminal is grounded. In addition, the first and second resistances R1 and R2 in series are arranged in parallel with the temperature sensing elements Rs and Rr to complete the bridge circuit. A junction between the first and second temperature sensing elements Rs and Rr, and another junction between the first and second resistances elements R1 and R2 are respectively connected to the aforementioned voltage converting circuit which in turn outputs absolute humidity amount to be detected.
Here, the junction between the two temperature sensing elements Rs and Rr is led to an inverting input terminal of an operation amplifier OP through an input resistance Rc of the OP, while the junction between the two resistances R1 and R2 is connected to a non-inverting input terminal of the operation amplifier OP. Connected to a point between the operation amplifier OP and the input resistance Rc of the OP is a negative feedback resistance Rf at its one end and the other end of which is connected to an output terminal of the operation amplifier OP. That is, the voltage converting circuit comprises OP, Rc and Rf.
The output voltage Vout outputted from the thus constructed operation amplifier OP is represented by the following formula (1): ##EQU1## where Av indicates an amplifier factor, represented by Av=Rf/Rc.
FIG. 2 shows a conventional circuit for detecting temperature, wherein, for example, thermistors having negative temperature coefficients are used as temperature sensing elements for the detector. As shown in FIG. 2, an element having negative temperature coefficients for example such as a thermistor decreases its resistance with an elevated temperature, therefore, if the thermistor is driven by a constant voltage power supply, the self-heating temperature of the thermistor sharply rises with the atmosphere temperature elevated, and in a worst case, this results in destruction of the element. To deal with this, a current limiting resistance Rd is usually inserted into between the power supply and the subject circuit. Other configurations are largely identical with the aforesaid case in which platinum resistances are employed. That is, in the humidity detector for detecting absolute humidity, the circuit includes two temperature sensing elements Rs and Rr, the first temperature sensing element Rs is exposed in the atmosphere, whereas the second temperature sensing element Rr is hermetically sealed. Both the first and second temperature sensing elements Rs and Rr are self-heated at approximately equal temperatures, and the first temperature sensing element Rs detects a change of heat transfer coefficient due to a humidity variation of the atmosphere by detecting a change in its own resistance value, while the second temperature sensing element Rr compensates for the temperature variation of the atmosphere.
In addition, the circuit is supplied with a power supply of a source voltage Eo for activating the humidity detector, and comprises two resistances, namely first and second resistances R1 and R2 which, together with the temperature sensing elements form a bridge circuit for detecting a ratio between the temperature sensing elements Rs and Rr, a current limiting resistance Rd and a voltage converting circuit (which will be described hereinafter) for converting the ratio between the temperature sensing elements Rs and Rr into a voltage. Specifically, the aforementioned two temperature sensing elements Rs and Rr are arranged in series, one terminal of which is connected to the current limiting resistance Rd, while the other terminal is grounded. A series of the first and second resistances R1 and R2 is connected in parallel with the two temperature sensing elements Rs and Rr to complete a so bridge circuit. The other terminal of the aforementioned current limiting resistance Rd is connected to the power supply of source voltage Eo. A junction between the first and second temperature sensing elements Rs and Rr, and another junction between the first and second resistances R1 and R2 are respectively connected to the aforementioned voltage converting circuit. The output of the voltage converting circuit and a voltage V.sub.T at a point between the first and second resistances R1 and R2 are inputted to and processed by an operation processing section 3, which in turn outputs an absolute humidity to be detected.
In this voltage converting circuit, like the aforementioned case in which the platinum resistances are used, a junction between the two temperature sensing elements Rs and Rr is connected to an inverting input terminal of an operation amplifier OP through an input resistance Rc of the OP, while another junction between the two resistances R1 and R2 is connected to a non-inverting input terminal of the operation amplifier OP. Connected to an intermediate point between the operation amplifier OP and the input resistance Rc of the OP is a negative feedback resistance Rf at its one end and the other end of the resistance Rf is connected to an output terminal of the operation amplifier OP. The output voltage Vout of this voltage converting circuit, i.e., the output of the OP, and a voltage V.sub.T at an intermediate point between the two resistances R1 and R2 are processed in the operation processing section 3 in which the output Vout is divided by V.sub.T for operational correction to make Vout/V.sub.T, which may be considered as absolute humidity output.
The output voltage Vout outputted from the thus constructed operation amplifier OP is represented by the following formula (2): ##EQU2## where Av indicates an amplifier factor as stated above, represented by Av=Rf/Rc, and Vf denotes a bridge circuit voltage expressed by Vf=Eo.times.Zs/(Zs+Rd), Zs is a combined resistance value expressed by Zs=(Rs+Rr).times.(R1 +R2)/(Rs+Rr+R1+R2).
Nevertheless, in the formula (2), the value Vf varies dependent upon the resistance values of the thermistors Rs and Rr which in turn change due to variation in temperature. So, in order to compensate for this change or to correct this value, there is a method disclosed in for example, Japanese Patent Application Laid-Open No. Sho 60-32288 in which the output value Vout is divided by V.sub.T, or a voltage at an intermediate point between R1 and R2. The thus defined value Vout/VT is calculated by the following formula (3), and is assumed as absolute humidity output. ##EQU3## where C is a coefficient.
As has been shown in formulae (1) to (3) heretofore, the ratio of the first temperature sensing element Rs to the second temperature sensing element Rr is converted into a voltage to obtain an absolute humidity.
FIGS. 3 and 4 show Rr - Rs characteristics of the platinum resistors and the thermistors, respectively, where the atmospheric temperature is elevated from Ta through Tb to Tc, and the absolute humidity is constant between Ta and Tb, and becomes increased gradually from Tc to Tb. As is apparent from the above formula, in an ideal state, or specifically when the relation Rs=Rr holds within a temperature between Ta and Tb, the output will be constant as long as the absolute humidity is unchanged even if the humidity is changed.
However, it is extremely difficult or impossible to choose a combination of elements Rs and Rr satisfying such a relation. In practice, the relation between Rs and Rr is represented by a linear formula, that is, Rs=A.times.Rr+B (A, B are coefficients). In some extreme cases, even the linear relation may not hold.
For these reasons, any change in temperature with the absolute humidity unchanged sometimes cause a change in output, that is, so called temperature drift occurs.
To deal with this, the following methods (a) and (b) have been taken as measures:
(a) A resistance for correction is added to either of Rs or Rr, as disclosed in Japanese Patent Application Laid-Open No. Sho 60-14149. In this method, A in the above relation Rs=A.times.Rr+B is assumed to be 1 and if B is positive, then a series resistance for correction having a value corresponding to B will be connected with the element Rr. On the other hand, if B is negative, then a series resistance corresponding to B for correction is to be connected with the element Rs.
(b) As is found in Japanese Patent Application Laid-Open No. Sho 60-203811, the current flowing through the elements will be controlled in accordance with variations of resistances Rs and Rr due to changes in temperature. For example, if the temperature of the atmosphere goes down, the current is increased so as to inhibit the temperature drop, whereas, if the temperature of the atmosphere goes up, the current is decreased so as to inhibit the temperature rise. With this control, the resistance variations of Rs and Rr can be made less, and thus the temperature drift will be inhibited.
Nevertheless, the above measures (a) and (b) have drawbacks as follows, and therefore neither of these measures would work sufficiently.
That is, the measure (a) can be applied only for the case of A=1, and furthermore it is difficult to set a resistance for correction for each of practical circuits. Therefore, in the practical case, a resistance value for correction is set up such that the temperature drift falls within an allowable range determined in advance. As a result, Rs and Rr should be selected in advance to some extent.
The measure (b) might exhibit some effects of correction, but a plurality of parameters are required upon determination of circuit constants for setting up the current limit. Moreover, the apparatus needs more parts giving rise to problems in cost and design.